<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .header {
      width: 900px;
      height: 200px;
      background-color: pink;
      margin: 100 auto;
      text-align: center;
    }

    /* 行内元素或者行内块元素水平居中给其父元素添加text-align:center 即可 */
  </style>

</head>

<body>
  <div class="header">
    <span>里面的文字</span>
  </div>

  <div class="header">
    <img src="down.jpg" alt="">
  </div>
</body>

</html>